1. Field of the Invention
The present invention relates to a wireless communications. More particularly, the present invention relates to adaptive calibration of a receiver system.
2. Description of the Related Art
With the advent and proliferation of digital communications systems, the need for low cost, high performance radio receivers continues to accelerate. These needs have prompted a strong interest in the development of direct conversion receiver architectures that implement a single conversion from a wireless link carrier frequency to a baseband frequency. The reduced complexity of direct conversion receivers holds great potential for reduced cost and increased performance.
Many modern digital communications systems use a form of quadrature modulation in which the wireless signal includes in-phase (I) and quadrature (Q) components which carry information with a relative phase offset of 90xc2x0. Typically, the in-phase and quadrature components are received using two distinct signal paths within a direct conversation receiver. Any difference in the gain or phase between the two paths corrupts the information in the signal. In addition, any DC offset or low frequency noise voltage which is generated by the receiver also corrupts the information in the signal.
In order to reduce the corruption, direct conversion receivers often employ an adaptive calibration mechanism. For example, prior art systems have been proposed which include a non-coherent adaptive calibration mechanism. However, such non-coherent adaptive calibration mechanisms exhibit high noise figures and, thus, do not accurately calibrate for receiver imperfections.
Therefore, there is a need in the art to develop an adaptive calibration system which provides accurate, low noise calibration.
A coherent adaptive calibration receiver and method is used to adjust for errors within a receiver, such as, for example, a direct conversion receiver used to receive a radio frequency signal. A series of first channel signal samples are summed with a first channel offset correction parameter to produce a corrected series of first channel samples. A series of second channel signal samples are summed with a second channel offset correction parameter and then multiplied by a gain imbalance correction parameter and summed with a product of the corrected series of the first channel samples and a phase error correction parameter to create a corrected set of second channel samples. When corrected, the first and second channels are orthogonal to one another.
The corrected series of first channel samples are multiplied with a first sinusoidal waveform to determine a first product. The corrected series of second channel samples are multiplied with a second sinusoidal waveform to determine a second product, the second sinusoidal waveform being 90 degrees out of phase with the first sinusoidal waveform.
The first product is filtered to determine a first channel gain imbalance measurement. The second product is filtered to determine a second channel gain imbalance measurement. A next value of the gain imbalance correction parameter is determined based upon the first and second channel gain imbalance measurements. In one embodiment, the multiplication and filtering are performed digitally.
Alternatively or in addition, the corrected series of second channel samples is multiplied with the first sinusoidal waveform to determine a third product. The third product is filtered to determine a phase error measurement. A next value of the phase correction parameter is determined based upon the first gain imbalance measurement and the phase error measurement.
In one embodiment, the invention is embodied in a receiver. The receiver has a first summer configured to sum a series of first channel signal samples with a first channel offset correction parameter to produce a corrected series of first channel samples. The receiver also has a first multiplier configured to multiply the corrected series of the first channel samples and a phase error correction parameter. A second summer is configured to sum a series of second channel signal samples with a second channel offset correction parameter. A second multiplier is configured to multiply an output of the second summer by a gain imbalance correction. A third summer is configured to sum an output of the first multiplier and the second multiplier to create a corrected set of second channel samples, wherein the second channel is orthogonal to the first channel. A third multiplier is configured to multiply the corrected series of first channel samples by a first digitized sinusoidal waveform. A fourth multiplier configured to multiply the corrected series of second channel samples by a second digitized sinusoidal waveform, the second digitized sinusoidal waveform being 90 degrees out of phase with the first digitized sinusoidal waveform. A first digital filter configured to filter an output of the third multiplier to determine a first channel gain imbalance measurement. A second digital filter configured to filter an output of the fourth multiplier to determine a second channel gain imbalance measurement. A calculator configured to determine a next value of the gain imbalance correction parameter based upon the first and second channel gain imbalance measurements.
In another embodiment, the invention is embodied in a receiver which has a first summer configured to sum a series of first channel signal samples with a first channel offset correction parameter to produce a corrected series of first channel samples. A first multiplier is configured to multiply the corrected series of the first channel samples and a phase error correction parameter. A second summer is configured to sum a series of second channel signal samples with a second channel offset correction parameter. A second multiplier is configured to multiply an output of the second summer by a gain imbalance correction. A third summer is configured to sum an output of the first multiplier and the second multiplier to create a corrected set of second channel samples, wherein the second channel is orthogonal to the first channel. A third multiplier is configured to multiply the corrected series of first channel samples by a first digitized sinusoidal waveform. A fourth multiplier is configured to multiply the corrected series of second channel samples by the first sinusoidal waveform to determine a third product. A first digital filter is configured to filter an output of the third multiplier to determine a first channel gain imbalance measurement. A second digital filter configured to filter an output of the fourth multiplier to determine a phase error measurement. A calculator configured to determine a next value of the phase correction parameter based upon the first gain imbalance measurement and the phase error measurement.